The present invention relates to an electrode configuration for a system that performs quadripolar transthoracic impedance monitoring.
Many implantable medical devices (IMDs) such as pacemakers, defibrillators, and others, are configured to measure transthoracic impedance via a function known as impedance minute ventilation (IMV) sensing. In some devices, the measured transthoracic impedance is used to adjust the rate of cardiac pacing, and many other uses of the measured transthoracic impedance are being implemented or contemplated, including pulmonary edema monitoring, sleep apnea monitoring and detection, respiration monitoring, measurement of mechanical cardiac functions, and others.
Transthoracic impedance measuring is performed by monitoring the voltage differential between pairs of spaced electrodes as current pulses are injected into leads connected to the electrodes. For example, a current pulse may be delivered between a ring electrode of a pacemaker and the conductive canister of the pacemaker, and a voltage differential may be measured between a tip electrode of the pacemaker and the conductive canister. This arrangement is referred to as a tripolar measurement system. It has been found that another desirable configuration of electrodes is one in which two independent sets of electrodes are used, known as a quadripolar measurement system. In a quadripolar system, two electrically isolated electrodes are provided at the canister of the device, which can add complexity to the design of the device.